Liquid treatment methods and apparatus

ABSTRACT

A liquid or water treatment apparatus comprising one or both of an electrodialysis cell and a cavitation unit. The cavitation unit generates cavitation in the liquid by flow of the liquid into a constriction where cavitation bubbles are formed and then to an outlet where cavitation bubbles implode, and the constriction includes an aperture formed by walls which are long and narrowly spaced in a plane normal to the flow direction. The electrodialysis cell is arranged with an inlet flow path for directing only part of a quantity of water to be treated through the electrodialysis cell, and an outlet flow path for returning a product of the electrodialysis cell to the remainder of the water.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a divisional of U.S. application Ser. No.12/446,133, filed Apr. 17, 2009, which is a national stage filing under35 U.S.C. §371 of PCT/GB2007/003903, filed Oct. 12, 2007, which in turnclaims priority to GB 0703598.3, filed Feb. 23, 2007 and to GB0620942.3, filed Oct. 20, 2006, all of which applications are herebyincorporated by reference in their entireties.

The present invention relates to the treatment of liquids, such aswater, in order to remove unwanted matter.

There are many situations in which the treatment of liquids is required,for example treatment of sewage water and potable water, treatment andprocessing of hydrocarbon liquids to break down undesirable long chainmolecules, treatment of waste water from exhaust gas cleaners. Morespecific water treatments are also used, such as treating ballast tankwater in ships as discussed below, and treating water to be used in fishfarming and the like. These latter uses require the break down ofunwanted matter in the form of organic matter, micro-organisms and thelike, generally in order to kill or disable them so that the treatedwater does not have undesirable effects in the environment into which itis released, or the environment in which it is put to use.

Ballast water is water transported by ships in the ballast water tanksor sometimes in other suitable spaces such as in cargo holds or in cargotanks. It is pumped into the tanks at a water “donor” location tocompensate for the changing of point of gravity as cargo and/or fuel isdischarged/consumed and hence to maintain stability. Correct ballastingis essential from a structural point of view and also used forperformance reasons in order to ensure proper propeller and rudderimmersion, proper bridge view as well as maintaining desired vesselmovement and handling characteristics. The ballast water is transportedto a water “recipient” location, generally at a point where the vesselis to be loaded with cargo, which is potentially outside thebio-geographic region of that of the ballast water origin. It may thenbe discharged as cargo is taken onboard. Ballast water may host a rangeof species including zooplankton, phytoplankton and bacteria. These maynot have natural predators at the point of discharge and may establishand reproduce at the new location causing significant problems for theenvironment, industry and human health.

In addition, corrosion in the ballast water tanks is caused due to thereaction of the ballast water and oxygen with the material from whichthe ballast water tanks are built. Currently, the application ofexpensive paints and coatings are used to prevent corrosion,representing a significant maintenance cost to the shipping industry.

It is known to treat water to clean it by the use of cavitation producedby ultrasonic excitation. For example, U.S. Pat. No. 5,137,580 disclosesa cleaning method which uses low and high frequency ultrasonic waves toform and grow cavitation bubbles. This treatment is carried out instatic tanks, which means that a continuous treatment process is notpossible and instead the tanks must be emptied and re-filled betweentreatments. As a consequence the treatment process is complicated andinefficient.

As well as the disadvantages of the use of a static tank, ultrasoniccavitation is less effective than other cavitation production methods,in particular hydrodynamic cavitation. This is because the bubble sizeand cloud density are not as large in cavitation generated by means ofultrasonic exposure compared to the bubble size and cloud density ofhydrodynamic cavitation. Thus, hydrodynamic cavitation possesses ahigher effective reaction volume compared to ultrasonic cavitation.

The use of venturi type constrictions to generate hydrodynamiccavitation is known, for example from U.S. Pat. No. 6,505,648. Thisdocument discloses a passageway for liquid flow with a reduced diameterportion forming a restriction which accelerates the flow of liquid andreduces the pressure to initiate cavitation in order to destroycontaminating organisms. A wire or rod across the restriction can beused to promote turbulence and further reduce the pressure. Downstreamof the restriction the passageway widens abruptly leading to collapsingof the cavitation bubbles.

However, this arrangement has been found to be ineffective, as not allorganisms in the liquid are affected by the collapsing cavitationbubbles, and as a consequence the water treatment process of the priorart does not treat all parts of the liquid.

Viewed from a first aspect the invention provides an apparatus for thetreatment of liquid, the apparatus comprising a cavitation unit, whereinthe cavitation unit generates cavitation in the liquid by flow of theliquid into a constriction where cavitation bubbles are formed and thento an outlet where cavitation bubbles implode, and wherein theconstriction includes an aperture that is elongate along a line normalto the flow direction.

By elongate it is meant that the aperture is narrow in a directionacross the line and is long in the direction along the line. Inparticular an aperture which is as long as possible is used, and whichhas a width of 5 mm or less, more preferably 1-2 mm as discussed below.The line may be a straight or a curved line. In preferred embodimentsthe aperture is an elongate slot or a narrow annulus as discussed below.

The use of an elongate narrow constriction allows cavitation to occur onthe whole of the cross-section of the flow of liquid. This is becausecavitation tends to initiate at an edge region of the constriction, andwith a long narrow aperture the greatest distance of any point in theflow path from the edge region is small. In contrast to the arrangementof the invention, a tubular constriction as in the prior art results incavitation that is concentrated about the edges of the tubular flow pathand thus is applied ineffectively and may not affect the whole of thefluid flow path. This is because unwanted matter in the fluid flowconcentrates in the centre of the tubular flow path, and hence is in theregion of least cavitation. In the present invention, by having agreater ratio of edge length to flow volume the amount of cavitationcompared to the cross-sectional area of the aperture is increased, andthe problem of unwanted matter flowing in a region of low cavitation isremoved. The aperture could thus be alternatively defined as an aperturehaving a cross-section with a large edge length in comparison to theenclosed area.

The aperture may be an elongate slot following a straight line but canbe any other elongate shape, such as an elongate aperture following azig-zag line or a curved line. The elongate aperture may follow a lineforming a close shape such as a square, circle or ellipse or so on.Preferably the aperture is a narrow annulus, i.e. an elongate aperturefollowing a circular line. The use of an annulus is preferred as thiscan be readily implemented in a pipe which has a circular cross-section,although as an alternative an oval or other shaped narrow slot could beused. It is beneficial for smooth flow of liquid for corners to beavoided.

The aperture may have a width of less than 5 mm more preferably lessthan 3 mm and in particular around 1 to 2 mm. The aperture is preferablyas long as possible along the line normal to the flow direction, and maybe for example at least 300 mm long where a width of 2 mm is used.

Where the aperture is an annulus a width as above and a diameter of 50to 100 mm may be used. This has been found to provide effectivecavitation for a pipe of that size, particularly when treating water tobreak down micro-organisms having a size of 10-50 μm, as a 1 to 2 mmslot ensures that organisms of this size are generally within a distanceof one bubble radius from collapsing bubbles as discussed above. Thepresence of organisms of this size in ballast water is restricted bylaw.

Preferably the cavitation unit is arranged to treat the liquid by usingcavitation to break down unwanted matter. Formation and implosion of thecavitation bubbles generates forces, temperature changes, and shockwaveswhich affect unwanted matter in the liquid. This unwanted matter may beorganic or inorganic waste, for example in waste water or in water to betreated in the production of potable water. The unwanted inorganicmatter may be long chain hydrocarbon molecules. In one preferredembodiment the long chain molecules are in waste water on ships in oilsor sludge or other hydrocarbon contaminants.

In a preferred embodiment the unwanted matter consists of water-borneorganisms, or micro-organisms. As discussed above, unwantedmicro-organisms are present in ballast water used in ships and thereforein a preferred embodiment the cavitation is used to treat ballast waterto break down such micro-organisms.

Preferably the cavitation unit is arranged such that at the point wherethe implosion of the bubbles begins the maximum distance from the edgeof bubble to the unwanted matter is less than the radius of the bubble.This can be achieved by selecting the width of the aperture in relationto the size of the unwanted matter. The implosion of bubbles isparticularly effective when it occurs in close proximity, and a distanceof less than the radius of the bubbles has been found to be effective toensure that unwanted matter, in particular micro-organisms, is brokendown by the cavitation effect.

An annular aperture may be created by a cavitation body in a pipe, inparticular a generally cylindrical obstruction in the pipe. Thus, theannulus is defined by the outer wall of the cavitation body and theinner wall of the pipe. The cavitation body may have a rounded end atthe inlet to the constriction, for example a section of a sphere. Theuse of a rounded end to channel the liquid into the constriction iseffective in providing the pressure decrease and flow velocity increaserequired to create cavitation. At the outlet end the cavitation body mayhave a tapered profile. This aids in increasing the pressure anddecreasing the flow velocity to promote implosion of cavitation bubbleswhilst minimising energy loss.

A wall of the aperture may have a non-uniform surface or roughenedsurface. The wall could be the pipe wall or the wall of the cavitationbody or both. Preferably the cavitation body has the non-uniform orroughened surface. This arrangement is easier to construct and allows astandard pipe to be used without modification to the inner pipe surface.The surface may be knurled, or a pattern of depressions or dimples maybe used. The use of an irregular surface promotes the cavitation effect.

The cavitation unit may comprise a plurality of constrictions such thatthe formation and implosion of bubbles is repeated. For example anannular aperture having an undulating profile along the flow directionmay be used, such as a wave-shaped or saw-toothed profile. This may beachieved by using a cavitation body having an undulating outer surface,which can be inserted into a standard cylindrical pipe.

Where the apparatus is used for the treatment of ballast water, or forany other use where a tank is filled or emptied a varying back pressureis applied by the tank to the apparatus. The effectiveness of thecavitation unit will be improved when the back pressure is maintainedabove a minimum level. Therefore, a pressure modification device ispreferably used to maintain the pressure to the cavitation unit atacceptable levels. For example a minimum back pressure of 1.5 bar or 2.5bar may be maintained, or a back pressure in the range of 2 to 2.5 barmay be maintained.

To ensure that all of the liquid to be treated is subject to the effectof cavitation, the distance between the walls of the aperture is limitedby the size of the unwanted matter to be broken down. Too large anaperture will not be effective, as the imploding cavitation bubbles willnot be close enough to the unwanted matter. As a result, the size of thecavitation unit and hence the maximum volume of water per unit timewhich can be treated is limited. Therefore, in a preferred embodiment,the apparatus includes a plurality of cavitation units in an array suchthat liquid can flow in parallel through a number of cavitation units.The cavitation units may be of different sizes. In one preferredembodiment the array comprises a large central support surrounded by aring of cavitation units. The cavitation units may be smaller than thecentral support. The large central support may be the same shape as thecavitation units. The large central support may act as an additionalcavitation unit. In another preferred embodiment many cavitation unitsof the same size in concentric rings are used.

The use of an array of cavitation units allows large flow volumes to betreated without compromising on the effectiveness of the cavitationtreatment. The apparatus may also include a manifold for channellingliquid equally to each cavitation unit to achieve the same flow rate ineach unit.

The apparatus may include a hydrophone that monitors cavitation toobtain cavitation data, wherein the cavitation data is used to controlsystem parameters of the cavitation unit. The hydrophone detects thepressure pulses caused by the cavitation unit, and the cavitation datamay be used to control parameters such as flow rate and pressure inorder to optimise the operation of the cavitation unit.

The apparatus may include an electrical treatment unit for treating theliquid prior to or following flowing it through the cavitation unit inorder to weaken or damage the unwanted matter in the liquid. Theelectrical treatment unit may apply an electrochemical effect,ionisation, a physical effect or any combination of the three. A watertreatment apparatus including an electrodialysis cell, which is believedto be inventive in its own right, is discussed below and may be used incombination with or as part of the water treatment apparatus describedabove.

The apparatus may include a gas injection unit, in preferred embodimentsthe injection unit is for the injection of nitrogen or oxygen containinggas into water. The oxygen containing gas may be air. Injection ofnitrogen reduces the amount of oxygen in the water and can therefore actto reduce corrosion and also to reduce weathering of corrosionprotection systems such as coatings and paints as oxidation is a causeof such weathering. This is useful when water is taken in to a ballasttank, as otherwise oxygen in the stored water can lead to corrosion orweathering of coatings. The tank could, for example, have a PU one layercoating or other one layer coating, but the water treatment is alsoadvantageous when other coatings are used.

When the water is returned to the environment, for example when emptyinga ballast tank, a gas mixture including oxygen, which may be air forexample, may be used to return oxygen to the water to avoid anydetrimental environmental impact. The gas injection unit may be a nozzlewithin a flow path of the liquid, and a static mixer may be placeddownstream of the nozzle. Alternatively, the gas injection unit mayincorporate combined gas and steam injection as discussed below inrelation to a water treatment apparatus including an injection unit,which is believed to be inventive in its own right.

Preferably, nitrogen gas is introduced into the water by separating apart of the water flow from the main flow, super-saturating this part ofthe water flow, and returning the supersaturated water to the main partof the water flow. The amount of water separated from the main flow maybe between 5%-30% by volume, preferably less than 15% by volume. Due tothe large amount of gas injected into a small volume, the flow followinggas injection may be characterised as a two-phase flow. In order toimprove mixing of the gas into the water, static mixer may be used inseparated water flow downstream of the point of gas injection. At thepoint of re-injection into the main flow, a static mixer may be appliedto facilitate mixing of the two-phase flow back into the main flow.

Viewed from a second aspect the invention provides a method of treatingliquid, the method comprising passing the liquid through a cavitationunit, generating cavitation by flowing the liquid into a constriction inwhich cavitation bubbles are formed and then to an outlet wherecavitation bubbles implode, wherein the constriction is provided by anaperture that is elongate along a line normal to the flow direction.

In preferred embodiments the method of the second aspect of theinvention includes method features corresponding to the preferredapparatus features discussed above.

It is also known to treat water by applying an electrical field. The useof an electrical field relies on a certain level of conductivity of thewater, which could be provided by impurities in the case of ‘fresh’water, or is provided by salts in sea water. In the case ofelectrochemical treatments, the presence of ions in the water that willrespond when exposed to an electrical current is also required, and onceagain these are provided by impurities and salts in the water. In thefield of ballast water treatment, CN 1736798 utilises an electrolytictreatment tank to generate oxidants and free radicals, and to kill ordeactivate organisms using the force of the electric field. The entiretyof the ballast water is treated by the electrolytic tank.

Electrodialysis is a known fluid treatment process which may be appliedto treat water for different purposes. The principle of the process isthat of ion-separation by applying an electric potential difference,either constant or in pulses, between two electrodes separated by amembrane, which may be ion-selective. One electrode will perform as ananode (positive charge) attracting negatively charged ions whilst theother will perform as a cathode (negative charge) attracting positivecharged ions. The fluid in the compartment between the membrane and theanode will become characterised by negatively charged ions with anexcess of electrons and may be referred to as the concentrate while thefluid in the compartment between the membrane and the cathode will becharacterised by the presence of positive ions with a shortage ofelectrons and may be referred to as the diluate. This configuration isknown as an electrodialysis cell.

In most practical electrodialysis processes, multiple electrodialysiscells are arranged into a configuration called an electrodialysis stack,with alternating anion and cation exchange membranes forming themultiple electrodialysis cells, generally between a single anode andcathode. The main known uses of electrodialysis are large scale brackishand sea water desalination and salt production, and small and mediumscale drinking water production. The process is also used in the processindustry for separation of certain contaminants such as heavy metals.

Electrodialysis systems can be operated as continuous production orbatch production processes. In a continuous process, diluate and/orconcentrate is passed through a sufficient number of stacks placed inseries to produce the final desired product quality. In batch processes,the diluate and/or concentrate streams may be re-circulated and/orcross-treated through the electrodialysis systems until the finalproduct or concentrate quality is achieved. In each case, the entiretyof the final product or products is treated using the electrodialysiscell, either in the diluate stream or in the concentrate stream.

U.S. Pat. No. 5,540,819 discloses an electrodialysis type cell forpreparing drinkable water from fresh water polluted with pathogenicmicro-organisms. The cell is divided by a permeable membrane into twocompartments containing an anode and a cathode respectively. Directcurrent is passed through the water between the anode and the cathode,and the water flows first through the anode compartment and then throughthe cathode compartment. All of the water to be treated is passedthrough the anode and then the cathode in sequence.

Viewed from a third aspect, the present invention provides a watertreatment apparatus comprising: an electrodialysis cell; an inlet flowpath for directing only part of a quantity of water to be treatedthrough the electrodialysis cell, and an outlet flow path for returninga product of the electrodialysis cell to the remainder of the water.

By returning water treated by the electrodialysis cell to the main waterflow, all of the water downstream of the electrodialysis cell ordownstream of the point of the outlet flow path will become affected byits characteristics without the need to expose all of the water to theelectrical treatment. The product of the electrodialysis cell has theeffect of disabling or killing micro-organisms, and can also have abeneficial effect on treating organic compounds in the water. Theinventors have found that by directing only a part of the water throughthe electrodialysis treatment cell and returning a product of theelectrodialysis cell to the water, an effective water treatment isachieved without the need to treat the entire water flow with theelectrodialysis cell. Thus, in the present invention only a small amountof water needs to be exposed to the electrodialysis treatment, ratherthan treating all of the water with electrodialysis as in the prior artsystems discussed above. This reduces the amount of electrical powerrequired to carry out the water treatment. In addition, the treatmentapparatus can be smaller compared to the prior art water treatmentsystems due to the comparatively smaller volume of water treated by theelectrodialysis cell in order to obtain an overall treatment effect onthe whole volume of water. Any effect on the flow rate of the main waterflow is also kept to a minimum.

The part of the water treated by the electrodialysis cell is preferablyseparated from the incoming water flow just prior to treatment and thenpassed through the electrodialysis cell as the remainder of the waterpasses by without being treated by the electrodialysis cell. Thus, theapparatus may include a main flow path, wherein the inlet flow path isarranged to separate a portion of the flow from the main flow path anddirect it through the electrodialysis cell. The apparatus may include aconnection from the outlet flow path to a main flow path, wherein theoutlet flow path introduces the product of the electrodialysis cell tothe main flow path.

The water which is not treated by the electrodialysis cell can beexposed to other treatments, which may be in parallel with theelectrodialysis treatment, for example a cavitation treatment or anitrogen injection treatment as discussed in more detail below.

Preferably less than 5% by volume of the total water flow into thetreatment apparatus passes through the electrodialysis cell, morepreferably less than 1% and yet more preferably less than 0.5%. Anamount of about 0.2% by volume is preferred, and depending onconditions, amounts as low as 0.05% or 0.01% could be used. It ispossible to manipulate the necessary flow volume by altering the currentused in the electrodialysis cell and the salinity of the water. Thus,depending on these factors and the particular application of thetreatment, the flow volume used can be larger or smaller.

In preferred embodiments, the invention is a ballast water treatmentapparatus. As discussed above, water treatment of this type isparticularly desirable for ballast water. Many existing water treatmentsare not suitable for ballast water treatment due to the high volume ofwater that needs to be treated in a short space of time. As only a partof the water needs to be passed through the electrodialysis cell, withthe remainder of the water not passing through the cell, the treatmentcan be applied to a much higher volume of water in a given time thanalternatives which require the entirety of the water to be directlyaffected by an electrical treatment.

The electrodialysis cell may be for producing a diluate stream and aconcentrate stream, with the product of the electrodialysis cell that isreturned to the water being composed of some or all of one or both ofthese streams. The product of the electrodialysis cell may simply be theconcentrate stream produced by the electrodialysis cell. However,preferably the product of the electrodialysis cell is a mixture of theconcentrate stream with at least a portion of the diluate stream. Theconcentrate stream contains an increased content of different oxidantsand the oxidants are particularly effective at killing or disablingmicro-organisms in the water when the product of the electrodialysiscell is returned to the main water flow.

After the electrodialysis treatment, the concentrate may have a lower pHthan the water prior to treatment, and the diluate may have a higher pH.Mixing the concentrate with some or all of the diluate therefore allowsthe pH of the product of the electrodialysis cell to be adjusted.

One or both of the concentrate stream and the diluate stream may beproduced by cross-treatment or recirculation. Cross-treatment, meaningsequential treatment (one sequence, two sequences or more) where theflow (from one compartment or from both compartments) between themembranes in the electrodialysis cell is re-circulated through theopposite compartment separated by the membrane of the cell, may beapplied to alter the characteristics of the concentrate and diluateand/or to reduce the quantity of the diluate and/or to simplify thefinal mixing of the two streams before re-injection to the main flow.Similarly the flow from the same compartments may be recirculated inorder to alter the characteristics of the concentrate and the diluatebefore final mixing and re-injection to the main flow.

In a preferred embodiment the concentrate stream and at least a portionof the diluate stream are mixed immediately after passing through theelectrodialysis cell. This may be done by removing a portion of thediluate stream, and then mixing the remainder of the diluate with theconcentrate stream. The amount of diluate removed may be between 40% and60% by volume.

In order to control the mixing ratio to keep the pH of the concentrateor mixed concentrate and diluate in a desired range the concentrate,diluate and/or mixed pH is monitored. The pH monitoring may be by meansof a pH electrode. The product of the electrodialysis preferably has apH of 1-5, more preferably a pH of about 3. A result of this is that thepH of the main flow, after the product of the electrodialysis cell isadded, is maintained within a range from 7.0 to 8.5, which is similar tosea water pH. The pH of the product of the electrodialysis cell may becontrolled by varying the amount of diluate added to the concentrate,for example by varying the amount of diluate removed prior to mixing.The pH of the concentrate is generally lower that the desired pH, andmixing with some of the high pH diluate may hence be used to increasethe pH of the product of the electrodialysis cell. Control of the pH mayalso occur by controlling the current or voltage supplied to theelectrodialysis cell, to thereby vary the strength of the resultantelectrodialytic effect and hence vary the oxidative strength of theconcentrate.

The apparatus may include a diluate removal flow path for removing apart of the diluate stream. To facilitate mixing of the concentrate andnon-removed diluate the apparatus may include a mixing area prior to theoutlet flow path. In one preferred embodiment, the mixing area is abuffer tank. Alternatively, the concentrate and diluate may be mixed asthey flow through the outlet flow path. Mixing may occur at the sametime as the concentrate stream and non-removed part of the diluatestream are mixed with the main flow, i.e. the product of theelectrodialysis cell may consist of two parts which are only mixed whenthese two parts are mixed with the rest of the water. In anotheralternative preferred embodiment, mixing is carried out within thebypass flow, for example with a static mixer or other mixingarrangement, and the flow is then reinjected after cavitation where thesystem pressure is lower due to the pressure drop over the cavitationunit. This arrangement avoids the need for a mixing tank or a dosagepump and also provide an opportunity to control and manage any gasbubbles arriving from the electrodialytic process as discussed in moredetail below.

Mixing may be promoted by a static mixer or turbulence inducing means inthe mixing area or in the outlet flow path.

The removed diluate may be re-injected to the water upstream prior tothe electrodialysis cell. If other treatment stages are included asdiscussed below, then the remainder of the diluate is preferablyre-injected prior to all treatment stages and even prior to filtrationand the suction side of the ballast pump if included. Re-injecting thediluate avoids the need to dispose of it, and it will react harmlesslywith impurities, unwanted matter and such like in the incoming untreatedwater. The diluate may advantageously be used as a cleaning agent, inparticular for the filtering processes if it is injected prior tofiltering, or stored and used directly to clean the filters.

The characteristics and amounts of concentrate and dilute reinjectedinto the main flow may be controlled by monitoring Oxygen ReductionPotential (ORP) and/or the consumption of Free Available Chlorine (FAC).The ranges for desired values of ORP may be 250-800 mV, more preferably300-500 mV. The immediate initial values of FAC following reinjection ispreferably between 2 and 4 ppm dropping to 0.1-0.4 ppm after a period of1 hour. The consumption of FAC is strongly dependent upon thecharacteristics of the water to be treated. To optimise the performanceof the electrodialytic cell, it is desirable to arrange a calibrationflow loop allowing presetting of current and mixing ratios prior toinitiating actual water treatment. When the ORP and/or FAC measuredvalues are outside the desired ranges, then the operation of theelectrodialysis cell is adjusted accordingly.

To direct the water flow, the apparatus may comprise conduits, pipes,baffles and the like. The electrodialysis cell may be integrated into aflow path for the main water flow, and thus the apparatus may include amain flow pipe or conduit for the main flow, with smaller pipes orconduits or the like for channelling a part of the main flow through thecell. Alternatively, the electrodialysis cell may be provided as astandalone unit which can be connected to an existing water conduit totreat the water therein. The treatment flow path may be formed by aconduit which is external to the main flow path. This allows an existingwater flow path to be easily adapted to include the treatment apparatusby the addition of an appropriate inlet and outlet junction. In thiscase, the treatment apparatus may include suitable pipes or conduits forconnection of the standalone unit to the existing conduit, along withvalves, dosage pump(s) and so on as required.

An independent source of brine may be used to augment the inputelectrolyte for the electrodialysis cell and increase its salinity. Thismight for example be brine produced as a by-product of freshwaterproduction or in a dedicated brine production plant, such as a reverseosmosis plant. A recirculating reverse osmosis plant may be used togenerate a saturated brine solution for use as an addition to the inputelectrolyte. The addition of brine or the like is required when thesystem is used to treat fresh water or weakly brackish water, asotherwise the electrical treatment will not be effective due to a lackof ions in the water. Brine may be also added to sea water with a lowsalt content in order to bring the salt content of the electrolyte to amore preferred level. At lower salt contents a larger electrical currentis required to achieve the same treatment effect with theelectrodialysis cell. Consequently, by increasing the salt content areduction in energy usage can be obtained. As an example, in the NorthSea a salinity of 25 parts per thousand or higher is typical, whereas inthe Baltic Sea surface waters have a much lower salinity, of perhaps 7parts per thousand. Preferably, brine is added to the input electrolyteto the electrodialysis cell to maintain a salinity of at least 25 partsper thousand.

Preferably, the water is stored for a period of time in a reservoir ortank after treatment. This allows time for the oxidants and reactivesubstances from the product of the electrodialysis cell to have fulleffect on any micro-organisms and other unwanted matter in the water. Ina particularly preferred embodiment, the invention is used in ship'sballast water treatment, wherein the water is treated as it is taken into the ballast tanks, and then it is stored in the ballast tanks beforedischarge. In this circumstance there is generally a reasonable time ofstorage as the ship moves from port to port before re-loading with cargoand discharging the ballast water. This time can be advantageously putto use in allowing the treatment by the product of the electrodialysiscell to take effect.

In order to allow a large volume of water to be treated, theelectrodyalsis is preferably provided by a plurality of electrodialysiscells in parallel, instead of one larger cell. This allows for a moremodular construction, and simplifies the construction of each cell byreducing the complexity and size of the parts.

The treatment may include gas injection. In preferred embodiments a gasinjection unit is provided for the injection of nitrogen and/or ofoxygen containing gas into water. The oxygen containing gas may be air.

Injecting oxygen can be used to return the oxygen content of the waterto an appropriate level before the water is discharged from the tank tothe environment, for example discharge out of a ballast tank into thesea.

Preferably, the treatment includes a gas injection unit for injectingnitrogen gas into the water. The nitrogenation of the water is thoughtto prolong the oxidant treatment, and also has a beneficial corrosionreduction effect as discussed above.

Nitrogen may be injected into all or a part of the water flow. Thenitrogen is preferably injected in sufficient amounts to ensure that thetreated water is super-saturated with nitrogen. In a preferredembodiment, a part of the water flow is separated from the main flow,and nitrogen is injected into this part. Preferably the part of thewater flow is less than 15% of the whole volume of water flow. When thenitrogenated water flow is reintroduced to the main water flow, a staticmixer may be used to promote mixing of the two water flows.

Turbulence resulting from the gas injection may be used to promotemixing of the product of the electrodialysis cell with the main flow.

The gas injection unit may be a nozzle within a flow path of the water,or a separated part of the water, and a static mixer may be placeddownstream of the nozzle. Passing the gas and water mixture through astatic mixer promotes nitrogenation of the water.

Alternatively, the gas injection unit may be as discussed below.

In a preferred embodiment, the apparatus includes a cavitation unit. Theuse of a cavitation unit produces a physical effect on anymicro-organisms and other living and non-living matter in the water andthus breaks down these unwanted elements. Preferably, the cavitationunit is placed to treat water before the product of the electrodialysiscell is returned to the treatment flow path. The cavitation treatmentcan thus be used to eliminate larger and more complex organisms, as wellas breaking down other unwanted matter, and in particular breaking downgroups or clumps of micro-organisms, with the product of theelectrodialysis cell then providing a final level of treatment thateliminates any remaining organisms, and is able to act more effectivelydue to the fact that larger sized organisms and groups of organisms havebeen broken down. Turbulence resulting from the cavitation treatment maybe used to promote mixing of the product of the electrodialysis cellwith the main flow.

The electrodialysis cell may advantageously be placed in parallel withthe cavitation unit. In this case, a small amount of the water flowpasses through the electrodialysis cell with the main part of the waterpassing through the cavitation unit. Preferably less than 5% by volumeof the total water flow into the treatment apparatus passes through theelectrodialysis cell, more preferably less than 1% and yet morepreferably less than 0.5%. An amount of about 0.2% by volume ispreferred, and depending on conditions, amounts as low as 0.05% or 0.01%could be used. The product of the electrodialysis cell is returned tothe main flow after the cavitation unit.

Although this means that a small proportion of the water is not treatedby the cavitation unit, the overall efficacy of the treatment is notsignificantly affected, and an advantage of having the inlet and outletof the electrodialysis cell either side of the cavitation unit is thatthe pressure drop over the cavitation unit provides a pressuredifference to drive the flow through the electrodialysis cell, avoidingthe need for a separate pump and making the system self powering in aclosed circuit. There are also added benefits in the resultant use of apressurised electrodialysis cell, and this minimises or avoids theproduction of gaseous hydrogen during the electrical treatment. Gaseoushydrogen production is a well know danger of electrodialysis of water.With the pressurised electrodialysis cell, although hydrogen may beformed it is always maintained in a safe, dissolved state.

With a pressurised electrodialysis cell, an appropriate membrane shouldbe selected, which allows the process to operate under pressure.Preferably the electrodialysis cells are constructed using a ceramicmembrane. Such membranes operate more effectively under pressure thanother types of membrane.

The cavitation unit may be a venturi type cavitation unit. In apreferred embodiment, the cavitation unit is a unit as discussed above.

Viewed from a fourth aspect the invention provides a method of treatingwater, the method comprising: flowing only a part of the water throughan electrodialysis cell, and returning a product of the electrodialysiscell to the remainder of the water.

In preferred embodiments the method of the fourth aspect of theinvention includes method features corresponding to the preferredapparatus features of the third aspect as discussed above.

In a particularly preferred embodiment the method is for the treatmentof ballast water, and the method comprises: treating the water used tofill the ballast tank in accordance with the fourth aspect of theinvention, injecting nitrogen into the water, storing the treated waterin the ballast tank, discharging the water from the tank and releasingit to the environment.

The method may include generating cavitation in the water as the waterenters or is discharged from the tank. Preferably cavitation is used totreat the water before the product of the electrodialysis cell isreturned to the water.

In a preferred embodiment, the method includes treating the dischargedwater by injecting an oxygen containing gas before releasing the waterto the environment. The oxygen containing gas may be air.

By treating the water as it enters and leaves the tank the risk ofstoring and releasing undesirable matter, in particular micro-organismsand other organic matter, is greatly reduced, as the various treatmentsteps result in such matter being broken down to a non-hazardous state.The injection of nitrogen into water which is then stored reducescorrosion of the ballast tank by reducing the amount of dissolved oxygenin the water. In addition, this reduces weathering of corrosionprotection systems such as coatings and paints as oxidation is a causeof such weathering. The injection of air as the water is releasedensures that sufficient oxygen is present in the released water to avoidadverse environmental effects from any remaining gas super-saturation.

Viewed from a fifth aspect the invention provides a water treatmentapparatus comprising a gas injection unit including an injection nozzleplaced within a water flow path, the injection nozzle being smallcompared to the size of the water flow path, wherein the injectionnozzle comprises a steam inlet, a gas inlet, a mixing region for mixingthe gas and steam, and an opening into the water flow path.

The use of steam improves the gas/water mixing and can therefore reducethe amount of gas required to achieve the desired effect. In addition,injected steam generates pressure pulses, which can affect organismspresent in the water and thus aid in breaking down unwanted organisms.By making the size of the injection nozzle small compared to the waterflow path, which may for example be a pipe or the like, the injectionnozzle does not obstruct the water flow.

Steam injection may be used to generate a higher speed jet downstream ofthe injection point. This accelerates the fluid and generates anadditional pumping effect within the water treatment apparatus.

Preferably the gas is mixed into the water in a jetting regime, and in apreferred embodiment this is achieved by the amount of injected steambeing higher than 150 kg/(m²s).

In a preferred embodiment the steam inlet comprises a nozzle, and thegas inlet comprises a passage about the steam inlet nozzle. The passagemay be an annular passage with outer wall of the annular passage formedby a pipe. The mixing part may be a chamber formed by continuation ofthe pipe beyond the open end of the steam inlet nozzle.

The opening into the water flow path may be simply an outlet nozzleformed by the end of the pipe, but in a preferred embodiment the outletnozzle includes a diverging profile where the mixture of gas and steamis expanded prior to entering the water flow path. The use of adiverging region avoids the risk of choking of the injected gas andsteam.

Viewed from a sixth aspect the invention provides a method of treatingwater by injecting gas into a water flow path using an injection nozzleplaced within a water flow path, the injection nozzle being smallcompared to the size of the water flow path, wherein gas and steam aresupplied from a steam inlet and a gas inlet and the gas and steam aremixed and injected into the water flow path.

Preferred embodiments of method of the sixth aspect may include featurescorresponding to the preferred features of the apparatus of the fifthaspect as discussed above.

Viewed from a seventh aspect the invention provides a water treatmentapparatus comprising an electrical treatment unit for applyingelectrical current to water and a cavitation unit, wherein water flowsin parallel through the electrical treatment unit and the cavitationunit.

By combining electrical treatment and cavitation any organisms and inparticular micro-organisms in the water, as well as other types ofunwanted matter, can be broken down more effectively. The parallel flowenables the pressure drop over the cavitation unit to drive waterthrough the electrical treatment unit.

In a preferred embodiment the apparatus is for the treatment of ballastwater, and is arranged to treat water being moved into or out of aballast water tank on a ship.

Preferably the apparatus includes a gas injection unit for the injectionof gas into the water after the electrical and cavitation treatment. Theuse of a gas injection unit allows the water to be treated with nitrogengas or with air in order to reduce or increase the oxygen content asdiscussed above, and to damage organisms by the effect of gassuper-saturation.

Nitrogen may be injected into all or a part of the water flow. Thenitrogen is preferably injected in sufficient amounts to ensure that thetreated water is super-saturated with nitrogen. In a preferredembodiment, a part of the water flow is separated from the main flow,and nitrogen is injected into this part. Preferably the part of thewater flow is less than 15% of the whole volume of water flow. When thenitrogenated water flow is reintroduced to the main water flow, a staticmixer may be used to promote mixing of the two water flows.

The apparatus of this aspect may include a first filter before theelectrical treatment unit to remove large bodies from the water. Afilter may be present after the cavitation treatment to remove smallerbodies from the water, in particular to remove broken downmicro-organisms and the like.

In preferred embodiments the cavitation unit is in accordance with theunit described in the first aspect and the preferred features of thefirst aspect. The electrical treatment unit preferably incorporates thefeatures of the apparatus of the third aspect and its preferredembodiments. Further, the gas injection unit may be in accordance withthe fifth aspect and preferred features thereof.

Viewed from an eighth aspect the invention provides a method of watertreatment comprising: splitting the water flow into two parts, treatingone part with an electrical treatment unit, treating the other part, inparallel, with a cavitation unit, and recombining the two parts.

The method may include injecting gas into the water.

One or more of the electrical treatment, generation of cavitation andgas injection are preferably in accordance with the methods of thefourth, second and sixth aspects of the invention respectively, and mayinclude the preferred features of those aspects as discussed above.

Thus, the electrical treatment may include treatments such as anelectrodialysis process, in which electricity is used to generatechemical reactions in the water, and these chemical reactions lead tofurther water treatment effects in the recombined water when theelectrically treated water is recombined with the other part of thewater.

Preferably the method is for the treatment of ballast water, and themethod comprises: treating the water used to fill the ballast tank asabove, with the gas injection unit injecting nitrogen, storing thetreated water in the ballast tank, discharging the water from the tank,optionally treating the discharged water by repeating the steps ofgenerating cavitation and/or injecting gas, with the gas injection unitinjecting air, and releasing the water to the environment.

By treating the water as it enters and leaves the tank the risk ofstoring and releasing undesirable matter, in particular micro-organismsand other organic matter, is greatly reduced, as the various treatmentsteps result in such matter being broken down to a non-hazardous state.The injection of nitrogen into water which is then stored reducescorrosion of the ballast tank by reducing the amount of dissolved oxygenin the water. In addition, this reduces weathering of corrosionprotection systems such as coatings and paints as oxidation is a causeof such weathering. The injection of air as the water is releasedensures that sufficient oxygen is present in the released water to avoidadverse environmental effects from any remaining gas super-saturation.

The apparatuses and methods of all of the embodiments described abovemay be for retrofitting to existing water treatment apparatuses, or tosystems in which it is desirable to add a water treatment apparatus.

Preferred embodiments of the invention will now be described by way ofexample only and with reference to the accompanying drawings in which:

FIG. 1A is a schematic of a water treatment system fitted with anexternal electrodialysis cell,

FIG. 1B is a schematic of a water treatment system fitted with anin-line electrodialysis cell;

FIG. 1C is a schematic of a water treatment system having anelectrodialysis cell in parallel with a cavitation unit,

FIG. 2 shows an embodiment of a cavitation unit,

FIGS. 3 to 5 a show various cavitation bodies with different means ofsecuring the cavitation body in a pipe,

FIGS. 6 to 9 show various alternative cavitation body shapes,

FIGS. 10 and 11 are plots of pressure against distance along thecavitation unit for a cavitation unit of the type shown in FIG. 8,

FIG. 12 shows two embodiments of arrays of cavitation units forincreasing the amount of water which can be treated,

FIG. 13 shows the arrays of FIG. 12 in cross-section,

FIG. 14 shows an arrangement of an injector in a pipe,

FIG. 15 is a diagram showing the detail of a gas/steam injector,

FIG. 16 shows an alternative injector arrangement,

FIG. 17 is a schematic of a static mixer in a pipe,

FIG. 18 shows a possible static mixer configuration,

FIG. 19 shows a partial cut-away view of an electrodialysis cell, and

FIG. 20 shows an alternative electrodialysis cell arrangement in partialcut-away view.

The embodiments of FIGS. 1A and 1B are intended for use as a ballastwater treatment system and is therefore described below in this context,but it will be appreciated that other uses for the described systemexist, and that the system can be adapted to suit differentrequirements.

FIG. 1A illustrates a first embodiment of a treatment system. The wateris filtered and then treated by a cavitation unit 10, a gas injectionunit 14 and an electrodialysis cell 8. This combination of treatmentscauses damage and death to the organisms in the water. As well asaffecting organisms in the water, nitrogen added to the water at theinjection unit 14 reduces the level of dissolved oxygen in the water andreduces the potential of re-growth of organisms as well as reducing theweathering of coatings and the speed of corrosion. Furthermore, thereduction in oxygen is thought to prolong the effect of oxidantsintroduced into the water via the product of the electrodialysis cellfrom the electrodialysis cell 8. By controlled atmosphere managementwhen the ballast tanks are empty by using nitrogen, these effects areenhanced further.

During filling of the ballast tanks, ballast water is pumped from thesea through an inlet pipe 1 by the use of the ship's ballast pump system2. After the pump 2, water flows through a pipe and is filtered througha first filter 4, which filters larger particles from the water. Theseform a sludge which is discharged at the point of ballast uptake.

Downstream of the first filter 4, a pressure booster may optionally beinstalled. The pressure booster can be used to maintain the level ofwater pressure needed for successful treatment in the units furtherdownstream.

Water then continues to flow into the cavitation unit 10. In thecavitation unit 10 hydrodynamic cavitation is induced by a rapidacceleration of the fluid flow velocity, which allows the fluid staticpressure to rapidly drop to the fluid vapour pressure. This then leadsto the development of vapour bubbles. After a controlled period of timewhich allows bubble growth, a rapid controlled deceleration thenfollows. This causes the fluid static pressure to rise rapidly whichcauses the vapour bubbles to violently collapse or implode exposing anyorganisms or the like in the water to the high intensity pressure andtemperature pulses, which breaks down the organisms in the water. Thecavitation unit 10 is described in more detail below in relation toFIGS. 2 to 13.

After the cavitation unit 10, a part of the water flows through theelectrodialysis cell 8. The remainder of the water is not treated by theelectrodialysis cell 8, and can simply continue to flow along a pipe orconduit to the later treatment stages. In the embodiment of FIG. 1A theelectrodialysis cell is fitted externally to the main flow conduit, andthus could be retrofitted to an existing treatment system.

In an alternative embodiment, instead of or in addition to the treatmentof incoming ballast water by the electrodialysis cell 8, another sourceof brine or saltwater 24 can be used as the input electrolyte for theelectrodialysis cell 8. This could be brine produced as a by-product offreshwater production or in a dedicated brine production plant, such asa reverse osmosis plant for example.

The source of brine or salt water can also be use to provide a boostingeffect to the salt content of the water when the system is used to treatfresh water or weakly brackish water. As discussed above, this enableselectrodialysis treatment of otherwise untreatable water, and also canbe used to reduce electricity consumption.

The electrodialysis cell 8, which is described in more detail below withreference to FIG. 19, produces a diluate stream 11 and a concentratestream 12. These two streams progress to a pH balancer or mixing unit13, which produces a product of the electrodialysis cell 17 that isdirected back into the main water flow, and depending on the compositionof the product 17, the mixing unit 13 may also give out a residue ofdiluate 18. The mixing unit 13 includes a pump or the like to controlthe amount of diluate 11 which is added to the concentrate 12 to formthe optimum product of the electrodialysis cell 17. In an alternativearrangement, as discussed above, the mixing unit may be within apressurised closed circuit, with pressure provided by the pressure dropover a parallel cavitation unit. In this case, no pump is required.

Downstream of the point of injection of the product of theelectrodialysis cell 17 there is a sampling and measurement point 15,which measures ORP and/or FAC and communicates the measured values tothe mixing unit 13. These measurements monitor the effect of theelectrodialysis cell 8 on the water and are used to control theoperational parameters of the cell and/or the mixing ratio applied.

The diluate residue 18 may be reinjected into the incoming water priorto all treatment steps, and preferably also before the filter and/or theballast water pump. Alternatively, it may be stored in a holding tank 25or ship's bilge water tank 26.

In the embodiment shown, the gas injection unit 14 treats the waterafter the product of the electrodialysis cell 17 is returned to the mainflow. However, in alternative embodiments the product 17 is returned tothe main flow downstream of the gas injection unit 14, with themonitoring unit 15 likewise downstream of the gas injection unit 14,monitoring the water conditions after the product 17 has been mixed in.

In the gas injection unit 14, nitrogen gas 16 is injected into theincoming water using a steam/nitrogen injector or a gas/water mixer inorder to achieve the desired level of nitrogen super-saturation in thewater, which kills organisms and reduces corrosion by reducing theoxygen level. This also prolongs the treatment effect of the oxidants inthe water. Embodiments of the injection unit 14 are described in moredetail below with reference to FIGS. 14 to 18.

Downstream of the treatment units, treated water is distributed by theship's ballast water piping system 23 to respective ballast water tanks.Here, excess gas is evacuated until a stable condition is achieved. Thisis regulated by means of valves integrated with the tanks ventilationsystem. These valves ensure stable conditions in the tank during theperiod the ballast water remains in the tank, in particular a high levelof nitrogen super-saturation and a low level of dissolved oxygen in thewater. Maintaining the level of super-saturation leads to an ongoingwater treatment both by the super-saturation itself and also by oxidantsintroduced by the electrodialysis cell 8. The treatment thus results intreated water that continues to kill or disable any surviving organismswhilst the water is stored in the ballast tanks and act as a preventivemeasure against re-growth.

Water is then left to rest in the ballast water tanks. When the ballastwater is discharged, water flows through a discharge treatment processthat returns the oxygen content of the water to an environmentallyacceptable level for discharge. The water is pumped from the ballasttanks and passes through the gas injection unit 14. This is used toreturn oxygen to the water as air replaces nitrogen as the injectiongas. Optionally, the water may be re-treated by the cavitation unit 10as it is discharged.

A second embodiment of a treatment system is shown in FIG. 1B. This isgenerally the same as the embodiment of FIG. 1A, but the electrodialysiscell 8 is placed in-line with the main flow rather than external to it.The water flows through an electrodialysis cell 8 such as that describedin more detail below with reference to FIG. 20. In this embodiment, themixing unit 13 may be installed as part of the in-line cell. As with theembodiment of FIG. 1A, the cell 8 produces a diluate stream 11 and aconcentrate stream 12 by treating a part of the water. The remainder ofthe water is allowed to pass the in-line cell 8 untreated and it ismixed with the product of the electrodialysis cell 17 as discussedabove. An external source of salt water or brine 24 may still optionallybe used.

As discussed above in relation to FIG. 1A, in an alternative to theembodiment of FIG. 1B, the gas injection unit 14 can inject gas upstreamof the point where the product 17 is mixed with the main water flow.This could be achieved by the injection of gas upstream of the inlet tothe electrodialysis treatment, or the gas injection unit 14 can injectgas into the water flow that is not treated by the electrodialysis cell8, and thus the electrodialysis treatment of a part of the water and thenitrogenation of the remainder of the water can occur in parallel.

A further arrangement of the components of the water treatment system isshown in FIG. 1C. In this system, water is pumped though an inlet 1 by aballast pump 2, and then filtered by a first filter 4 as discussedabove. The flow of water is then split, with a small proportion of about0.2% by volume being routed through an electrodialysis cell 8, and theremainder passing through a cavitation unit 10. It should be noted thatthe electrodialysis cell operates as discussed in relation to FIGS. 1Aand 1B, and that other features relating to the electrodialysis cell 8as shown in FIGS. 1A and 1B may be present, such as the holding tank 25or ship's bilge water tank 26.

After the electrodialysis cell 8 and cavitation unit 10 the product 17of the electrodialysis cell 8 is mixed with the water treated by thecavitation unit, and this water then flows to the gas injection unit 14.In this embodiment, the gas injector is shown acting on only a part ofthe water flow instead of being fully in line with the water flow as inFIGS. 1A and 1B. It should be noted that the arrangements of the gasinjector 14 can be interchanged between the embodiments of FIGS. 1A, 1Band 1C. With the arrangement shown in FIG. 1C nitrogen is injected intoa part of the water flow, which is less that 15% by volume of the total,perhaps 10% by volume. The nitrogen is injected in a sufficient amountto ensure that the water is super-saturated with nitrogen. When thenitrogenated water flow is reintroduced to the main water flowturbulence from the combining flows, or alternatively a static mixer, isused to promote mixing of the two water flows and ensure that the entirewater flow is nitrogenated to a sufficient level to have the desiredwater treatment effect.

After the introduction of nitrogen, the treated water passes throughpipe 23 to the ballast tank, where it is stored. As with the embodimentsabove, when it is necessary to discharge the water from the ballasttank, the system is arranged to route the outgoing water through atreatment process including introduction of oxygen to re-oxygenate thewater, and optionally a repeated cavitation treatment.

FIGS. 2 to 13 show embodiments of the cavitation unit 10. Cavitation isa process of nucleation in liquids, induced by the fall of the localstatic pressure below the local vapour pressure at constant temperature.The fall of the local static pressure causes a liquid to boil, whichleads to a development of small bubbles of vapour. An increase of thestatic pressure causes these vapour bubbles to collapse.

A high pressure is generated in the last phase of the bubble collapsewhich literature assumes to lead to the emission of a shock wave. Theshock wave can be launched with a shock velocity of almost 4000 m/s anda shock amplitude decay faster than 1/r. The pressure pulse in thevicinity of a bubble has been reported to be up to 1 GPa in some cases.High local temperature spots with a temperature up to 7800K have alsobeen reported in the literature as a consequence of the cavitationbubble collapse. If cavitation occurs near a rigid surface, the bubblescollapse asymmetrically, often forming a fast-moving water jet. This jetmay create surface damage and possibly lead to jet-induced tissuedamage, when an organism or the like is in a vicinity of the collapsingbubble. The jets, high pressures and temperatures created by collapsingcavitation bubbles have a destructive effect on micro-organisms in waterby causing damage to the tissue of the organisms which consequentlyleads to the death of the organisms.

However, fast moving jets and high pressure and temperature spots occurin close vicinity of the collapsing bubble and therefore only affectorganisms, which are close enough to the bubble at the time of collapse.In order to utilise the energy, released at the implosion of the bubblemore effectively, exposure of the organisms should be targeted. Forwater treatment applications in general and ballast water treatmentapplications in particular, this may be achieved by inducing cavitationin or near a small gap through which treated water flows.

In the preferred cavitation unit 10, hydrodynamic cavitation is inducedby a rapid controlled acceleration of the fluid, which allows the fluidstatic pressure to rapidly drop to the fluid vapour pressure, this thenleads to the development of vapour bubbles. After a controlled period oftime which allows bubble growth, a rapid controlled deceleration thenfollows. This causes the fluid static pressure to increase rapidly whichcauses the vapour bubbles to violently collapse or implode, exposing theorganisms to high intensity pressure and temperature pulses.

The geometry of the cavitation unit 10 has been designed to utilise theobserved synergetic effects of the presence of surfaces in relation tobubble generation and the importance of the proximity of the implodingbubble to the target object. Different configurations of the design havebeen developed with different characteristics, including repeatablecavitation, all based on the same preferred geometric design.

The cavitation unit 10 creates circumferential cavitation about acavitation body in the form of a torpedo-like shape 30 as shown in FIG.2. The torpedo like-shape 30 is advantageous as it is generallycylindrical and can be easily fitted into a pipe 31, such as a ballastwater pipe or any pipe carrying water. The particular shape of thetorpedo 30 can be designed to provide the appropriate pressure drop inorder to induce the maximum cavitation given any particularcircumstance. The preferred shapes optimise bubble growth and sizedistribution, in particular in order to bring micro-organisms close tothe cavitation bubble collapse zone to achieve the maximum destructiveeffect of the cavitation unit 10.

The basic parameters for the manipulation on the torpedo 30 of FIG. 2are the length A, radius B, the gap width C, angle D, radius of thetorpedo nose E and length of the torpedo middle section F. Theseparameters may be altered to achieve particular cavitationcharacteristics. In general, the cavitation unit 10 is arranged toprovide an elongate area where cavitation is instigated by constrictionof the flow path, which in this embodiment is achieved by a narrowannular passage. Liquid flows from left to right as viewed in FIG. 2.The velocity of the water is increased and the pressure decreased at thetorpedo nose E, and hence cavitation bubble growth is initiated. Thebubbles grow along the torpedo middle section F, and then implode as thewater pressure increases again.

FIGS. 3 to 5 a are four different versions of the cavitation body 30showing how assembly of the torpedo unit into the ballast or the wastewater pipe 31 can be achieved.

FIG. 3 illustrates fins 32 applied to fit the torpedo 30 in the pipe 31.The length, angle, height, depth and number of fins 32 can be varied.

FIG. 4 illustrates pins 33 applied to fit the torpedo in the pipe. Thepins 33 are attached to the torpedo 30 at the front and rear of theunit. The length, diameter, position and number of the pins 33 can bevaried.

The fins 32 and the pins 33 are designed to provide a secure fit of thetorpedo 30 in the water pipe 31. Furthermore, the design is such thatthey affect the developed cavitation area as little as possible in orderfor the torpedo unit 30 to provide the maximum possible cavitation.

FIG. 5 illustrates both pins 33 and fins 32 applied to fit the torpedo30 in the pipe 31. Four pins 33 are attached to the torpedo unit in thefront of the unit and four fins 32 are attached to the torpedo unit atthe back of the unit.

FIG. 5 a shows a torpedo 30 of an alternative design, where the torpedo30 is held in place by lugs 52, which slot into supports. These lugs 52remove the need for pins or fins and hence avoid any obstruction to thefluid flow other than the torpedo itself, and hence promotes a moreuniform fluid flow.

FIGS. 6 to 9 are various alternative designs of the torpedo 30. Thereare two different plain versions of the torpedo 30 in FIGS. 6 and 7,which provide two different possibilities of assembly of the unit intothe water pipe. In particular, FIG. 7 shows a torpedo 30 with a blunttrailing edge 34, which may be used for easier mounting of the rear endof the torpedo 30 to the pipe 31. The plain version of the torpedo 30induces cavitation by causing a single rapid drop and rise in pressureover the unit and a cavitation collapse zone at the end of the middlesection of the torpedo 30.

In addition to the plain version there are two more versions of thetorpedo unit as shown in FIGS. 8 and 9. These two versions can have therear part of the unit designed as shown in FIG. 6 or as in FIG. 7.

FIG. 8 shows a torpedo with an undulating profile 35 that inducesmultiple cavitation zones by multiple drops and rises of pressure. Withthe torpedo 30 of FIG. 8 cavitation collapse zones are achieved behindeach of the rippled sections of the unit. This is advantageous as thedestructive effect of the cavitation is repeated on each section.

FIG. 9 shows a torpedo 30 with a section of irregular surface 36. Thesurface could be a knurled surface or a dimpled surface for example. Theuse of an irregular surface 36 enhances the generation of bubbles due tothe cavities on the surface of the torpedo unit 30.

FIGS. 10 and 11 show the vapour pressure profiles for two differentdesigns of the undulating surface version of the torpedo 30 shown inFIG. 8. The graphs have pressure on the vertical axis and the horizontalaxis represents distance along the cavitation unit. The low pressurepoints are below atmospheric pressure P. The graph in FIG. 10 shows thepressure profile for a torpedo 30 with two ripples, i.e. one less thanthe torpedo shown in FIG. 8. The graph in FIG. 11 shows the pressureprofile for a torpedo 30 with three ripples, such as the torpedo 30 ofFIG. 8.

The number of ripples on the middle section, the position of the rippleson the middle section of the torpedo and the minimum and the maximumdiameter of the ripples can be varied in order to achieve the desiredcavitation effect. The parameters are chosen in order to achieve themaximum number of cavitation zones and maximum area/volume ofcavitation.

It will be appreciated that the features of FIGS. 6 to 9 could beapplied to torpedoes 30 having the same general shape shown in any ofFIGS. 3 to 5 a, as well as to other shapes of cavitation body.

FIG. 12 shows two different versions of arrays 37 of multiple cavitationunits 10. These illustrate how a standard cavitation unit 10 may bemultiplied with a number of units in order to provide different rangesof flow rate capacities. The versions should allow more control andflexibility than a single torpedo version, which is limited by the needto maintain a narrow annulus in order to maximise the effect ofcavitation. By having multiple cavitation units 10 the efficacy of thecavitation effect can be maintained without restricting the use ofhigher flow rates.

In one version a large torpedo 10′ is positioned in the middle of thewater pipe 31 with smaller torpedoes 10″ placed around a large torpedo.The smaller torpedoes 10″ produce cavitation as discussed above. Thelarge torpedo 10′ can simply be for supporting the smaller torpedoes 10″and in this case would not have any cavitation function, although thefront and rear ends of the torpedo are profiled to direct water flow tothe smaller torpedoes 10″. Alternatively the large torpedo 10′ couldoperate in the same way as the smaller torpedoes 10″ and the torpedoes10 as discussed above in order to produce cavitation to treat the water.The diameter of the new unit X can be equal, smaller or larger than thediameter of the water pipe Y. The ratio of X/Y depends on the desiredwater flow rate and the number of torpedo units used in the multipleunit versions.

In an alternative version a number of small torpedoes 10″ are assembledinto a water pipe 31 without the use of a larger unit 10′. In theembodiment shown the smaller torpedoes 10″ form two concentric ringsabout a central small torpedo 10″

FIG. 13 shows cross-sectional views of the arrays of FIG. 12, withhigher or lower numbers of the smaller cavitation units 10″. Differentnumber of small torpedoes 10″ may be placed into a unit, depending onthe flow-rate requirements.

FIG. 14 shows a gas injector unit 14 comprising an injection nozzle 43positioned in a pipe 31. Nitrogen 16 or air 29 is injected into thewater flow path. Steam 38 may also be injected to improve the mixing ofthe gas 16, 29 into the water as discussed below. Gas can be injectedinto water using a gas/steam injector nozzle 43 as shown in FIG. 15, gasinjection into a constriction nozzle in a pipe 31 as shown in FIG. 16 ora gas/water static mixer 44 as shown in FIGS. 17 and 18. The features ofFIGS. 16 to 18 are known gas mixing systems that can be utilised in thepresent water treatment apparatus. In the present water treatmentprocess, nitrogen 16 is injected through the injector unit 14 during thefilling part of the treatment and the air 29 is injected during thedischarge part of the treatment.

As discussed above, the injection of gas, and in particular theinjection of nitrogen, can be carried out on only a part of the waterflow instead of injecting gas into the entire water flow. The gasinjector units described herein could of course be placed in the mainflow path, as in FIGS. 1A and 1B, or could be placed in a separate flowpath, along which only a part of the water flows before being returnedto the main flow, as in FIG. 1C.

In preferred embodiments steam 38 is injected at the same time as thegas 16, 29 as in FIG. 14. The use of steam 38 aids mixing of the gas 16,29 into water and reduces the amount of gas 16, 29 required. Inaddition, steam injection results in a generated pumping effect. This isbecause steam injection leads to a higher speed jet behind the injectionpoint that accelerates the fluid, creating an additional suction, whichis felt in the system as an additional pumping effect. Further, injectedsteam generates pressure pulses, which affect any unwanted matterpresent in the water, and provide a physical water treatment effect inaddition to the effect of the gas/steam injector on the dissolved gascontent of the water.

To achieve best effects of steam and gas injection, some conditions forinjector design must be followed. These involve an appropriate regime ofsteam condensation, the generation of a high speed two-phase jet,appropriate gas bubble sizes and also avoiding choking of the injector.

Steam condenses in water in different regimes depending on the amount ofinjected steam, the temperature difference between the steam and water,the diameter of the steam injector, and the amount of impurities/gas inthe steam. There are three main regimes of condensation of steam intowater. These are chugging, bubbling and jetting. Different regimesaffect the mixing of gas into water differently. The most desirableregime for the purpose of the mixing is a jetting regime. In general,the jetting regime is achieved effectively when the amount of injectedsteam is higher than 150 kg/(m²s).

In a two-phase jet, steam injected into water generates a jet downstreamof the injection. The jet enhances the mixing of the gas into water andprovides an additional pumping effect.

If the exit diameter of the injector is too small then steam starts tochoke in the injector. Choking of the steam reduces the mixing processand should be avoided.

Gas can be mixed with the steam prior to injection. Gas in the steamreduces the condensation rate because the bulk of the gas is pushed tothe surface of the condensing steam which must therefore condensethrough the gas layer. However, if the amount of the gas is too high,steam condenses inside the mixing chamber of the injector, which reducesthe mixing effect.

The range of gas bubble sizes also affects the mixing performance. Therange is dependent on the amount of injected steam, the amount ofinjected gas, the temperature of the water and the gas, the pressure andthe condensing regime etc.

FIG. 15 shows a preferred design of a steam/gas injector nozzle 43. Theparts are shown in close up view. The nozzle structure 43 of FIG. 15 issmall in comparison to the size of the pipe 31 and would be placed atthe centre of the pipe 31 as shown in FIG. 14.

The injector nozzle 43 consists of three main parts. In an inlet region40 steam 38 and gas 29, 16 are supplied into the injector nozzle 37. Theinjector nozzle 43 is formed of a small pipe 45 within a larger pipe 46.Steam 38 is supplied through the small pipe 45 in the centre of theinjector nozzle 43 and gas 29,16 is supplied in the area defined aroundthe steam pipe between the larger pipe 46 and the small pipe 45. Thewater flow is outside of the larger pipe 46.

Following the inlet region is a mixing region 41 where steam 38 and gas29,16 are mixed. In FIG. 15 the mixing area is formed by the small pipe45 ending, and the larger pipe 46 continuing. The gas/steam mixture isinjected into the water flow through a diverging region 42 of theinjector nozzle 43. The total length of the injector, lengths of thedifferent regions of the injector, the diverging angle, the radius ofthe main pipe 46, the outlet of the injector and the radius of the steampipe 45 can be varied in order to achieve the desired mixing effect.

Water is present in the injector nozzle 43 prior to the steam and gassupply. Steam and gas are also not fully mixed in the mixing chamberprior to the injection into the water. To achieve full mixing adifferent injector design could be used with a converging region inplace of the diverging region 42. However, the present design assures noor little choking in the injector nozzle 43, which increases the mixingperformance.

FIG. 16 shows an alternative gas injector unit 14. In this arrangementgas 29,16 is injected into a constriction in the pipe 31.

FIG. 17 shows the use of a static mixer 44. The static mixer 44 is fixedin the centre of the water flow. Gas 29, 16 is supplied in the waterflow upstream of the mixer and is mixed with water by turbulence duringthe flow through the mixer 44. The gas may be supplied using nozzlearrangements as discussed above, or by alternative conventional systems.

An example of a static mixer 44 is shown in FIG. 18. An angularturbulent highly-mixed multiphase fluid flow through the static mixer 44is achieved by connecting separate plates together as shown. The outercircumference of the static mixer is circular allowing the unit to befitted into the pipe 31.

The operation of the electrodialysis cell 8 will now be explained.Embodiments of the structural arrangement of electrodialysis cells 8 aredescribed below with reference to FIGS. 19 and 20. For each of theseembodiments, the chemical processes are basically the same. As discussedabove, electrodialysis is an electro-membrane process where ions aretransported through ion permeable or ion selective membranes in a fluidsystem. In the simplest implementation of an electrodialysis cell asingle membrane or membrane pair is placed between two electrodes. Anelectric charge established by applying a voltage between two electrodesallows ions to be driven through the membrane provided the fluid isconductive. The voltage is applied by power connection points of aconventional type, which are not shown in the drawings. The twoelectrodes represent respectively the anode and the cathode. Theelectric charge creates different reactions at the different electrodes.At the anode, the electrolyte will have an acidic characteristic whilstat the cathode, the electrolyte will be characterised by becomingalkaline. Membranes used in electrodialysis are chosen for the abilityto create selective transport. Thus, this allows the alkaline solutionto be kept separate from the acidic solution.

Various reactions which occur in an electrodialysis cell where theincoming electrolyte is ballast water taken from a ballast waterpipeline (i.e. sea water) are shown in Table 1 below. Thus, the rawelectrolyte can have characteristics such as conductivity, which alterthe effect of the electrodialysis process.

TABLE 1 Reactions at the anode: Reactions at the cathode: 2Cl⁻ − 2e →Cl₂2H₂O + 2Na⁺ + 2e → 2NaOH + H₂ 2H₂O − 4e → 4H⁺ + O₂ 2H₂O + 2e → H₂ + 2OH⁻Cl₂ + H₂O → HClO + HCl O₂ + e → O₂ ⁻ HCl + NaOH → NaCl + H2 O₂ ⁻ + H⁺ →HO₂ Cl⁻ + 2OH⁻ − 2e → ClO⁻ + O₂ + H₂O + 2e → HO₂ ⁻ + OH⁻ H₂O 3OH⁻ − 2e →HO₂ ⁻ + H₂O O₂ + 2H₂ + 2e → H₂O₂ + 2OH⁻ HO₂ ⁻ − e →HO₂ H⁺ + e → H^(•)OH⁻ − e →OH^(•) H^(• +) H^(• →) H₂ OH^(• +) OH^(• →)H₂O₂ OH^(• +)OH^(• →)H₂O₂ HClO + H₂O₂ →HCl + O₂ + H₂O H₂O₂ + OH^(•) → HO₂ + H₂OClO⁻ + H₂O₂ →¹O₂ + Cl^(•) + H₂O H₂O₂

 H⁺ + HO₂ ⁻ H₂O₂+ OH⁻

 HO₂ ⁻ + H₂O OH⁻ + HO₂ ⁻

 O₂ ²⁻ + H₂O O₂ ²⁻ + H₂O₂ → O₂ ⁻ + OH⁻ + OH OH + H₂O₂ →H₂O

Table 2 below illustrates typical properties for an acidic solutionproduced at the anode and an alkaline solution produced at the cathode.The acidic solution forms the concentrate stream and the alkalinesolution forms the diluate stream.

TABLE 2 pH FAC (ppm) ORP (mV) Acidic solution (at the anode) 2-3.5400-800 1100-1200 Alkaline solution (at the cathode) 11-12.5 — 800-900

The two separated streams are mixed in a ratio providing a product ofthe electrodialysis cell and optionally a residue with typicalcharacteristics shown in Table 3.

TABLE 3 pH FAC (ppm) ORP (mV) Concentrate 7.5-8.5   500-800 750-800Residue 11-12.5 800-900

In order to tailor the chemical characteristics of the two streams,cross-treatment may be applied. This may constitute of an arrangementallowing all of or a portion of one or both streams to be re-injected atthe entrance to the opposite compartment to the compartment from whichit arrived from. Thus, the concentrate stream produced by the anodecould be cross-treated by re-injection into the cathode side of thecell. The characteristics of the stream(s) expressed by pH, ORP and FACmay by this method be further tailored and enable the amount of residualdiluate after mixing to be reduced if mixing is applied in addition.

Recirculation of respective flows from respective compartments may alsobe applied in order to tailor the characteristics of the two streams inorder to improve the desired characteristics of the end productfollowing mixing, and also with the aim of reducing the amount ofresidual diluate.

The mixing ratio will depend on the “quality” of the raw electrolyte,the size of the electrodes and the power applied.

The product 17 of the electrodialysis cell 8 is introduced into the mainflow, after the cavitation treatment. The nitrogen injection can occurbefore or after the product 17 is added.

The ratio between concentrate/residue and ballast water flowing throughthe line is controlled by monitoring the ORP and FAC upstream at amonitoring station 15. The characteristics and amounts of concentrateand dilute reinjected into the main flow are controlled by monitoringOxygen Reduction Potential (ORP) and/or the consumption of FreeAvailable Chlorine (FAC). The range for desired values of ORP is 300-500mV. The immediate initial values of FAC following reinjection ispreferably between 2 and 4 ppm dropping to 0.1-0.4 ppm after a period of1 hour.

FIG. 19 illustrates an embodiment of an electrodialysis cell 8 that canbe used to treat water in the system of FIG. 1A. Water is passed throughan annular passage formed between a solid cylindrical anode 47 and ahollow cylindrical cathode 48. To form the electrodialysis cell 8 an ionexchange or ion selective membrane 49 is placed between the anode 47 andthe cathode 48. The cathode 48 and the membrane 49 are shown in cut-awayview. As the water forming the incoming electrolyte passes through theannular passage, an acidic concentrate solution 12 is formed at theanode side of the membrane 49 and an alkaline diluate solution 11 isformed at the cathode side of the membrane 49. A portion of the diluatestream 11 may be separated off, and all of or the remaining part of thediluate stream 11 is mixed with the concentrate stream 12 to form theproduct of the electrodialysis cell 17 as discussed above.

An alternative embodiment of an electrodialysis cell 8 for in-line useis shown in FIG. 20. The electrodialysis cell 8 includes a plate shapedanode 47 and cathode 48, sandwiching a membrane 49. The cell 8 is housedwithin a pipe 50, shown in partial cut-away view, and the pipe 50transports all of the water flow in the direction shown by the arrows.The majority of the water will pass by on either side of theelectrodialysis cell 8, with only a part of the water entering theelectrodialysis cell 8 and flowing along between the anode 47 and themembrane 49 or between the cathode 48 and the membrane 49. On the anodeand cathode sides reactions occur when electrical current is passedthrough the electrodes and thus a diluate stream 11 and concentratestream 12 are produced at the cathode and anode sides respectively asdiscussed above.

In order to control the composition of the product of theelectrodialysis cell 17 that is returned to the main water flow, adiluate removal flow path 51 is provided. The amount of diluate removedfrom the electrodialysis cell 8 is controlled based upon the measuredORP and/or FAC levels downstream of the electrodialysis cell 8. Asdiscussed above, in some cases all the diluate will be mixed with theconcentrate stream 12, such that there is no diluate removed. Theconcentrate stream 12 and the remainder or all of the diluate stream 11are then reintroduced to the main water flow at the end of theelectrodialysis cell 8. These two parts together form the product of theelectrodialysis cell 17 although they are not mixed together until theyare mixed along with the main water flow as the flow through and aroundthe electrodialysis cell 8 progresses beyond the end of the cell 8.

To allow efficient treatment of higher flow rates by the electrodialysisequipment, a number of electrodialysis cells 8 can be placed in an arrayto treat water in parallel. Although it is possible to increase the sizeof the electrodialysis cell 8 to achieve the same effect, theconstruction of larger cells becomes more complicated, and so multiplesmaller cells are preferred. In addition, the use of smaller cellsallows for a more modular construction, which means that the same partscan be produced in bulk and combined together to produce systems forvarious different flow rates of ballast water or other water to betreated.

1. A water treatment apparatus comprising: an electrodialysis cell; aninlet flow path for directing only part of a quantity of water to betreated through the electrodialysis cell, and an outlet flow path forreturning a product of the electrodialysis cell to the remainder of thewater, wherein the electrodialysis cell is for producing a diluatestream and a concentrate stream, and the product returned to theremainder of the water includes at least some of the concentrate stream.2. An apparatus as claimed in claim 1, wherein the apparatus is aballast water treatment apparatus.
 3. An apparatus as claimed in claim1, wherein the product of the electrodialysis cell is a mixture of theconcentrate stream with at least a portion of the diluate stream.
 4. Anapparatus as claimed in claim 3, wherein the ratio of diluate toconcentrate is controlled to maintain the pH of the product of theelectrodialysis cell within a selected range.
 5. An apparatus as claimedin claim 1, comprising a water flow path between a point at which theinlet flow path directs a part of the water to the electrodialysis celland a point at which the outlet flow path returns the product of theelectrodialysis cell to the remainder of the water, and a cavitationunit in the water flow path for subjecting the remainder of the water toa cavitation treatment in parallel with the electrodialysis treatment.6. An apparatus as claimed in claim 5, wherein the water flow throughthe electrodialysis cell is driven by a pressure drop occurring acrossthe cavitation unit.
 7. An apparatus as claimed in claim 1, comprising adiluate removal flow path for removing at least a part of a diluatestream produced by the electrodialysis cell from the electrodialysiscell.
 8. An apparatus as claimed in claim 7, wherein the removed diluateis re-injected into untreated water upstream of the inlet flow path. 9.An apparatus as claimed in claim 1, comprising a source of brine,wherein the apparatus is arranged to use the source of brine to maintainthe salinity of the water directed into the electrodialysis cell above apredetermined minimum level.
 10. A method of treating water, the methodcomprising: passing only a part of the water through an electrodialysiscell, and returning a product of the electrodialysis cell to theremainder of the water, wherein the electrodialysis cell produces adiluate stream and a concentrate stream, and the product of theelectrodialysis cell includes at least some of the concentrate stream.11. A method as claimed in claim 10, wherein the method is a method oftreating ballast water.
 12. A method as claimed in claim 10, wherein theproduct of the electrodialysis cell is a mixture of the concentratestream with at least a portion of the diluate stream.
 13. A method asclaimed in claim 12, comprising controlling the ratio of diluate toconcentrate to thereby maintain the pH of the product of theelectrodialysis cell within a selected range.
 14. A method as claimed inclaim 10, comprising subjecting the remainder of the water to acavitation effect prior to introduction of the product of theelectrodialysis cell to the water.
 15. A method as claimed in claim 14,comprising using a pressure drop over the cavitation unit to drive waterflow through the electrodialysis cell.
 16. A method as claimed in claim1, comprising removing a portion of the diluate from a diluate streamproduced by the electrodialysis cell, and directing the removed diluateinto untreated water upstream of the inlet flow path.
 17. A method asclaimed in claim 10, comprising using a source of brine to maintain thesalinity of the water directed into the electrodialysis cell above apredetermined minimum level.
 18. A vessel comprising a ballast watertreatment apparatus as claimed in claim
 1. 19. A method of providing aballast water treatment apparatus in a vessel, comprising providing anapparatus as claimed in claim
 1. 20. A method as claimed in claim 19comprising retrofitting the apparatus to an existing vessel.